1. Field of the Invention
The invention relates to geared transmissions wherein ratio changes in gearing are effected by ratio change clutches under the control of clutch shift forks carried by shift fork rails.
2. Background Art
A multiple ratio power transmission for motor vehicles typically is classified as either an automatic transmission or a transmission with positive engagement clutches (e.g., synchronizer clutches) for achieving ratio changes in transmission gearing. Automatic transmissions typically include fluid pressure operated clutches and brakes that are selectively engageable under the control of a transmission controller to effect ratio changes in the gearing. A transmission with positive engagement ratio change synchronizer clutches typically include clutch sleeves that are mechanically shifted in the direction of the axes of individual gearing elements in the transmission to establish and disestablish a mechanical connection between the gear elements and a power delivery shaft. The clutch sleeves carry clutch teeth that are selectively engageable with clutch teeth carried by an adjacent gear.
Shift forks carried by shift rails typically are used to shift the clutch sleeves into and out of engagement with clutch teeth carried by the individual gears. Each of the clutch sleeves is slidably connected, usually through spline teeth, to a clutch hub connected to a power delivery shaft on which a gear is mounted.
A shift linkage mechanism actuates the shift rails in the direction of the axis of the power delivery shaft. During a ratio change, a master clutch is disengaged so that a ratio change can occur as the master clutch interrupts torque delivery from the power source, such as an internal combustion engine, to a power output shaft for the transmission.
A transmission with positive engagement ratio change clutches, especially a transmission that has a number of ratio steps, may have a plurality of shift rails, wherein each shift rail actuates a shift fork for a torque delivery gear. In a power transmission mechanism with multiple ratios that require multiple shift rails, packaging of the shift rails within a transmission housing presents a design problem because of the need for space within the transmission housing for accommodating the shift rails. The requirement for increased space, of necessity, increases the size of the transmission housing itself. This may introduce packaging problems for assembling the transmission within the limited space available in an automotive powertrain assembly. Further, an increased size of the housing would increase the weight of the powertrain as well as the manufacturing cost of the powertrain.
Typical examples of automotive transmissions that use positive engagement ratio change clutches may be seen by referring to U.S. patent publication US2008/0188342 and U.S. Pat. No. 5,105,674. The '342 publication, which is assigned to the assignee of the present invention, has multiple shift rails for controlling four positive engagement ratio change clutches. In the case of the design of the '674 patent, a single shift rail can be used, but provision must be made for both angularly adjusting the shift rail about the shift rail axis and for shifting the shift rail in the direction of the shift rail axis to activate multiple shift forks for five forward driving ratios. This complicates the overall design of the gear ratio shifting mechanism of the transmission. Further, a single shift rail design is not readily adaptable for automatic pressure operated shift rail actuators. It is better adapted for use with a gear shift tower having a driver controlled gear shift lever mounted on the transmission housing, whereby the gear shift lever effects both rotary motion and linear motion of the shift rail.